Sadly the hot water tank in my 1975 argosy is cracked and must be changed...
The rest of the model no.G6A2 still functions, the pilot and the burner seam to work just fine... Im a bit confused on what to do, should i change just the tank and how easy is that?? or should I buy and all new unit?

BTW. when i light up the unit the was no water in it, the previous owner had bypass the water from the cracked tank... the flame after a while just went out... hope i did not break anything "Newbie".

I have a '74 Argosy and would recommend a new "basic" unit.
The most difficult part will be the gas line. The newer models are not in the same place. But close.
The Atwood 6 gallon model is the replacement for the old Bowen.

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You may need a tubing bender and flaring tool for the gas line. It's been 4 years since I replaced mine. As I recall I replaced the gas line from the water heater to the gas shut off valve below the trailer.
Also wrenches to fit the gas line fittings. Always use 2 wrenches when loosening or tightening gas line fittings.
The existing cover will work.
I installed wooden blocks between the inner and outer skins around the opening in order to get a firm attachment of the new heater.
Caulk will be needed to seal around the edges. I used a product called LEXELL. It works great and is still pliable after 4 years.
Do not remove the styrofoam from the water heater. It should be installed with this in place. It serves as insulation to prevent heat loss.
This would be a convenient time to install a bypass valve system for winterizing.

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You are welcome.
I worked in Baie-d'Urfe in the early 90's. spent almost a year there.
I am somewhat familiar with the area.
My wife is Canadian. Met her in Vancouver over 20 years ago.
It's probably more than you wanted to know.
If you have more questions. Help is here.

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__________________Knowledge: "A gift to be shared. A treasure to receive."